A network enables the transmission of messages between its nodes. In a network, not all nodes of the network are directly connected to all other nodes. A message from a sending node to a receiving node must therefore often be forwarded via one or several intermediate nodes in order to arrive at a receiving node from a sending node. The path from the sending node via the intermediate node to the receiving node is referred to here as a path or route.
A routing method is used to select a suitable path for a message from a large number of theoretically possible paths in the network. The routing method firstly determines at least one, expediently however a plurality of path candidates, along which the message could be transmitted. A path distance value, a so-called route metric, is subsequently assigned to the path candidate in each instance. The path distance value is a measure of the quality of a path candidate. The path distance value is in turn usually determined from link distance values, which are in turn a measure of the quality of the links in the respective path candidate. The direct individual connection of the two nodes in the network is referred to here as a link.
Usage costs for a link in the path or the number of links in a path can enter into the path distance value for instance. It is also possible to enter the values for a transmission quality along the path candidate or a link in the path candidate or values for the transmission speed of the path candidate or a link in the path candidate. The path candidate with the optimal path distance value is selected below as a path. The message can now be transmitted along this path.
The methods for determining the path distance value are referred to as routing metrics. A known routing metric is an ETX (Expected Transmission Count). With the routing metric ETX, the path with which the number of transmissions to be expected is the lowest is selected. Transmissions are understood here to mean both transmissions as well as retransmissions. A first transmission is the transmission of a packet via a link. A retransmission takes place if the first transmission was not successful. The first transmissions and the retransmissions are treated equally in the case of ETX.
Retransmissions are nevertheless disadvantageous in that they may require more time than first transmissions. ETX is therefore disadvantageous in that it does not determine the optimal path for certain types of data transmissions in certain scenarios. Such data transmissions may be Voice over IP (VoIP) or video telephony for instance. Other examples of such types of data transmissions are all types of data transmissions, in which packet repetitions have negative effects on the quality of data transmission, and are therefore sensitive to repetitions.